This invention relates to methods, compositions, and apparatuses for the monitoring and controlling of paper sheet characteristics on a creping process. As described at least in U.S. Pat. Nos. 7,691,236, 7,850,823, 5,571,382, 5,187,219, 5,179,150, 5,123,152, 4,320,582, and 3,061,944, in the tissue manufacturing process, a paper sheet is dried and creped on a heated drying cylinder, termed a Yankee or Yankee dryer. Creping is a process in which a steel, bimetallic, or ceramic blade (called a doctor blade) is impacted into the paper sheet, thus compressing the sheet in the machine direction (MD), creating a folded sheet structure. Creping breaks a large number of fiber-to-fiber bonds in the sheet, imparting the qualities of bulk, stretch, absorbency, and softness which are characteristic of tissue. The amount of adhesion provided by the coating adhesive plays a significant role in the development of these tissue properties. Often adhesive materials are used to coat the Yankee surface in order to help the wet sheet adhere to the dryer. This improves heat transfer, allowing more efficient drying of the sheet. Most importantly, these adhesives provide the required adhesion to give good creping of the dry sheet.
The Yankee coating also serves the purpose of protecting the Yankee and creping blade surfaces from excessive wear. In this role, the coating agents provide improved runability of the tissue machine. As creping doctor blades wear, they must be replaced with new ones. The process of changing blades represents a significant source of tissue machine downtime, or lost production, as creped product cannot be produced when the blade is being changed. Release agents, typically blends of hydrocarbon oils and surfactants, are used in association with the coating polymers. These agents aid in the uniform release of the tissue web at the creping blades, and also lubricate and protect the blade from excessive wear.
In the creping process as the paper sheet is removed from the dryer surface macro and micro folds are formed that appear sharper on the air side of the sheet, while these folds are more broken up and less sharp on the Yankee side. The resulting structures formed appear as repeating bars whose MD length (machine direction) tend to be shorter than the CD (cross direction) length. Property changes to the sheet as a result of the creping process include bulk, stretch, softness, and absorbency all increasing with strength decreasing. In particular, the tactile surface smoothness of the sheet is strongly linked to the crepe structures formed on the sheet. All of these properties are critical to the manufacturer for quality control, product development, and machine troubleshooting. Controllable variables impacting the crepe structure include coating chemistry, crepe ratio (Yankee speed/reel speed), sheet moisture level, and creping blade geometry and age. Other process variables such as furnish, forming dynamics, and fabric also affect the creping process, but are not as easily controlled.
Previous methods of evaluating creped sheet characteristics and surface topography are described at least in U.S. Pat. Nos. 5,654,799 and 5,730839, US Published Patent Application 2005/0004956, International Patent Application WO 2007/024858, and Published Articles: The Measurement of Surface Texture and Topography by Differential Light Scattering, E. L. Church, Wear, 57 (1979), 93-105, Tactile Properties of Tissue with Moire Interferometry, Lidnsay, J., Bieman, L., 1997 Engineering & Papermakers: Forming Bonds for Better Papermaking Conference, Oct. 6, 1997, TAPPI, Image Analysis to Quantify Crepe Structure, Archer, S., Furman, G., and W. Von Drasek, Tissue World Americas 2010 Conference, Mar. 24-26, 2010, Miami, Fla. USA, Reprint R-974.
Monitoring the crepe structure formed in the sheet provides insight on the machine running conditions and product quality. Manufacturers recognize this point and will routinely evaluate the sample by counting macro crepe structures using an ocular device with or without image storage capability. The procedure uses an oblique light source perpendicular to the CD of the sheet, and results in scattering light from the crepe structures to visually form alternating light and dark areas. The bright areas represent crepe bars and are manually counted over a unit length scale to determine the number of crepe bars per inch (CBI) or cm. Tracking the CBI number allows the manufacturer to assess product quality and machine running conditions. For example, a reduction in the CBI number could be linked to operating conditions such as an aging doctor blade or a moisture profile change affecting the sheet adhesion. Once the problem is identified, proper corrective action can be taken to restore the desired product quality.
However, unlike tensile strength, stretch, basis weight, caliper, and moisture, which are quantitative measurements, crepe bar counting is a qualitative subjective measurement. The subjectivity in manual CBI measurements results from the complex topography of the creped sheet being composed of macro and micro structures, free fiber ends, and fractured structures. As a result, CBI analysis is dependent on the technicians experience and skill to identify and interpret what is and is not a crepe bar structure. This lack of standardization and repeatability in manual CBI measurements is a limitation in using the information for process control decisions and product quality assessment.
Thus there is clear need and utility for methods, compositions, and apparatuses for the uniform consistent and accurate measurement of creped paper sheet properties. The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR §1.56(a) exists.